The present invention pertains to heat exchangers, and, in particular, to a heat exchanger incorporating an encapsulated phase change material to provide a heat storage.
Heat exchangers with a variety of different constructions are known in the art, and are employed to accomplish various functions. One known heat exchanger, which is disclosed in U.S. Pat. No. 5,220,954, is a phase change material heat exchanger. In that design, a quantity of phase change material is encapsulated within a container encased within a tube. The annular space defined between the container and the tube is selectively sectioned such that heat exchanging fluid is routed in a specified, circuitous fashion through such space to obtain increased heat energy transfer between the passing fluid and the phase change material.
Heat exchangers of this type, if used properly, are especially advantageously employed in the task of making use of available energy. The current press for energy can most economically be met by using the untapped capacity of the existing electric generating plants, which untapped capacity essentially lies in the off-peak production capabilities of such plants. The transfer of off-peak produced energy through storage in heat exchangers to on-peak periods would provide improved shaping of the load profile for such plants. However, while useful, existing phase change material heat exchangers still have room for improvement both in the rate by which heat energy can be transferred to and from the phase change material, and the more efficient use of electricity to achieve a heat transfer to the phase change material.
Thus, it would be desirable to provide a heat exchanger which overcomes these and other shortcomings of the prior art.
The present invention provides a phase change material heat exchanger that has a space around the phase change material holding container divided into circuitous passageways for heat exchange fluid, and further includes a series of heat energy transfer elements, such as resistance heated rods, suspended within the interior volume of the container and embedded within the phase change material located therein. The heat energy transfer elements are suitable for directly introducing heat energy to achieve melting of solid phase change material, or, in certain circumstances, for withdrawing heat energy to promote the solidification or freezing of liquid phase change material within the heat exchanger container.
In one form thereof, the present invention provides a heat exchanger including a container, a phase change material within the container, a tube around the container and including an inlet and an outlet and defining with the container a flow path between the inlet and the outlet for a working fluid for transferring heat energy with the phase change material, and a plurality of means within the container for transferring heat energy with the phase change material.
In another form thereof, the present invention provides a heat exchanger including a container, a phase change material within the container, a tube around the container and including an inlet and an outlet and defining with the container a flow path between the inlet and the outlet for a working fluid for transferring heat energy with the phase change material, and at least one electrical resistance heated heat transfer element circuited with an electrical power source. The at least one heat transfer element is in contact with the phase change material within the container and is heatable to a sufficiently high temperature to initiate melting of the phase change material from its solid phase to its liquid phase.
In still other forms of the present invention, a heat exchanger for a phase change material having a solid density greater than its liquid density is provided having a container holding the phase change material, a tube surrounding the container to define an annular space therebetween, means for connecting the tube in fluid communication with a first source of heat exchange fluid to allow heat exchange fluid to flow through the annular space to exchange heat with the phase change material, and at least two divider walls extending between the tube and the container across the annular space to divide the annular space into at least an upper flow passageway and a second flow passageway, the upper flow passageway for receiving heat exchange fluid from the first source at a temperature sufficiently cool to initiate freezing of liquid phase change material in the container so that the phase change material is frozen from the top so that the newly-formed solid phase change material falls by gravity to a lower portion of the container to displace liquid phase change material to an upper portion of the container, the second flow passageway below the upper flow passageway for receiving heat exchange fluid from the upper flow passageway to flow in counterflow relationship with the heat exchange fluid flowing in the upper flow passageway. In one of such other forms of the present invention, the heat exchanger also includes at least one electrical resistance heated heat transfer element circuited with an electrical power source and positioned within the container to be in contact with the phase change material, and heatable to a sufficiently high temperature to initiate melting of the phase change material from its solid phase to its liquid phase. In a different one of such other forms of the present invention, the heat exchanger also includes a plurality of heat energy transfer elements positioned within the lower portion of the container and extending through the phase change material, the plurality of heat energy transfer elements each having an elongate shape and heatable to a sufficiently high temperature to initiate melting of the phase change material from its solid phase to its liquid phase so that the phase change material is melted from the bottom as it moves to the lower portion of the container to cause newly-formed liquid phase change material to be displaced to the upper portion of the container.
One advantage of the heat exchanger of the present invention is that it allows extra energy to be stored for subsequent use instead of merely dissipated and wasted.
Another advantage of the present invention is that it allows off-peak energy to be captured and diverted to on-peak uses.
Another advantage of the present invention is that it facilitates the process of liquefying phase change material in a heat exchanger with off-peak electrical energy.
Another advantage of the present invention is that the embedding of additional heat energy transfer elements within the phase change material achieves a direct contact with the phase change material that promotes an efficient heat transfer.
Still another advantage of the present invention is that heat energy transfer elements may be located within the phase change material to melt that material from the bottom, and the energy stored in the melted phase change material is released to a working fluid flowing circuitously in an annular space of the heat exchanger from the top of the heat exchanger to the bottom, whereby the releasing of energy to the working fluid results in a freezing of the phase change material that can be remelted by the heat energy transfer elements.